Continuous wave multiple frequency metal detectors have significant advantages over their single frequency counterparts in terms of ground rejection and target discrimination. They also have significant advantages over the pulse induction detectors in terms of target sensitivity and target discrimination. However, for these advantages to be realised, it is essential that the simultaneous transmission at several frequencies is done accurately and efficiently.
Some preferred requirements of a good multi-frequency transmitter are to have high efficiency, to be constructively simple and of low cost, to maximise the power transmitted at the required frequencies and to minimise the power transmitted at any other frequencies. In addition, the ability to independently change any of the transmitted frequencies, to keep the transmitted signals at comparable amplitudes, and to employ simple means to generate the signals to be transmitted is also desirable.
For optimum operation of the metal detector, a good selection of the nominal values of the transmitted frequencies is required. It is known that a linear progression on a logarithmic frequency scale (or a geometric progression on a linear frequency scale) is a good choice because for both targets and grounds, relevant features can be best observed on a logarithmic frequency scale. For good target discrimination and good rejection of ground signals it is desirable to have as many frequencies as possible. However, for a given amount of transmitted power, the more frequencies are used, the less power is transmitted on each frequency. At the same time, the amount of noise received is proportional with the number of frequencies used; therefore the signal-to-noise ratio decreases with increasing the number of frequencies. It was found that using four to eight frequencies gives a good compromise between detection depth (sensitivity) and target discrimination while being able to reject soils that are both magnetic and conductive.
If the detector uses digitally intensive techniques, as described for example in WO2006/021045, then there are a number of alternatives that can be used to achieve efficient multi-frequency transmission, including for example, a class B (or AB) amplifier in conjunction with tuned circuits, a class D (switching) amplifier driving the transmit winding directly, a switching (square or rectangular) wave transmitter driving the transmit winding directly, etc. Each alternative has advantages and disadvantages.
For example, the class B (or AB) amplifier in conjunction with tuned circuits compensates the lack of efficiency of the linear amplifier with the recirculation of the current offered by the tuned circuit. While constructively simple, this approach has the drawback that the operating frequencies must be matched to the resonant frequencies of the tuned circuit, which are determined by the circuit elements (inductors and capacitors) and therefore susceptible to accuracy and drift issues. This also limits the ability to vary the operating frequencies around their nominal values, which is at times necessary for avoiding external interference. Additionally, changes in the inductance of the transmit winding due to ground mineralisation produce significant phase shifts between the excitation and the resultant current, requiring greater accuracy for amplitude and phase corrections. If the driving frequencies are generated digitally, the cost of this solution is increased by the need to provide one or more digital-to-analogue converters (DAC). An advantage of this solution is that, if the amplifier and the DAC (if required) have reasonably low distortion, the transmitted signals are spectrally clean, owing to the filtering effect of the tuned circuit.
The solution based on the class D (switching) amplifier theoretically comes close to the ideal: high efficiency, flexibility in the choice of the operating frequencies (no restrictions due to tuned circuits), and low distortion. A class D amplifier can have either analogue or digital input. In the analogue input case it might require a DAC (if the transmitted signals are generated digitally) and its construction is more complicated, but can achieve relatively low distortion. In the digital input case, the construction is simpler, but the amplitude resolution is limited, causing distortion and in-band spurious signals. In both cases the output low pass filter (reconstruction filter) is a critical circuit element, as it can introduce frequency dependent amplitude and phase variations. The filter also controls the amount of switching frequency, its harmonics and spurious signals leaking into the transmit winding and further on into the receive winding and receive circuit. The compromise between desirable amplitude and phase versus frequency characteristics and switching frequency suppression depends on the damping of the low pass filter, which introduces losses. These losses are in addition to those inherent to the switching action of the power stage. When these variables are taken into account, the class D amplifier is not as attractive as it initially appears.
On the other hand, switching transmitters for metal detectors are known to have many desirable characteristics, like simple construction, low power dissipation and relatively low electromagnetic compatibility issues. Most of them transmit a repetitive multi-period waveform which has the property that its fundamental and/or some of its harmonics have higher magnitude. However, with such waveforms, it is relatively difficult to insure that strong Fourier transform components of comparable magnitude only occur at a few selected frequencies and that all other harmonics have low magnitude. Also, the frequencies used in operation must be integer multiples of the fundamental, which can be limiting at times.
There are examples in the prior art, like UK patent application GB 2 423 366 A and many others where two signals with the same frequency but different phases and/or duty cycle are applied to the inputs of two half-bridge amplifiers, whose outputs are in turn connected the load. This arrangement generates a 3 level waveform with reduced amount of energy at higher frequency harmonics. Obviously, this is an improvement, but single frequency operation is inadequate for high performance metal detection. In the U.S. Pat. No. 4,311,929, the two independent half-bridge switching amplifiers are driven with two signals with different frequencies, effectively summing the two signals across the load. This also generates a three level waveform, but the patent does not extend the method to more than two frequencies.
The current invention provides a switching transmitter of a new and novel configuration that overcomes or at least substantially ameliorates the problems associated with existing transmitters for metal detectors.